chore: improve v1 release readiness

[KyleTryon]

Summary

• Add a Docker image smoke test that runs the built image, checks /api/health, and verifies the SPA root responds.
• Refresh stale pre-v1 metadata and docs around supported providers, pnpm version, and credential persistence.
• Split editor export UI and codec registration into lazy chunks to reduce the editor route's initial payload.
• Extract editor subtitle state into a focused hook so EditorScreen is easier to maintain.

Impact

This improves release confidence before v1 by testing the production container path, tightening public-facing project metadata, reducing initial editor bundle size, and making subtitle state easier to reason about.

Validation

• pnpm lint
• pnpm test
• pnpm build
• pnpm knip